Miniature button alkaline silver oxide-zinc cells have gained wide acceptance in the battery industry for many applications because they have a high energy density per unit volume. However, one of the major disadvantages of divalent silver oxide-zinc cells is that they discharge at two different potentials. Silver oxide-zinc cells using monovalent silver oxide as the only active cathode material will have a theoretical unipotential discharge at about 1.57 volts but the capacity in milliampere hours per gram of monovalent silver oxide is substantially lower than the capacity of divalent silver oxide. On the other hand, silver oxide-zinc button cells using divalent silver oxide as the only active starting cathode material will discharge at a first potential at about 1.7 volts across a 300-ohm resistor for 40 hours, for example, and then drop to approximately 1.5 volts for an additional period of time of about 70 hours. Thus monovalent silver oxide cells have the advantage of discharging at a unipotential plateau with the disadvantage of having a rather low capacity while divalent silver oxide cells have the advantage of having a rather high capacity but the disadvantage of discharging at two distinct voltage plateaus. Divalent silver oxide has about 1.9 times more capacity per gram than monovalent silver oxide and about 2 times more capacity per unit volume than monovalent silver oxide.
Many cells or battery applications, particularly transistorized devices such as hearing aids, watches and the like, require a substantially unipotential discharge source for proper operation and, therefore, cannot use the dual voltage level discharge which is characteristic of divalent silver oxide cells.
Consequently, many methods have been proposed for obtaining a unipotential discharge from a divalent silver oxide cell. One method disclosed in U.S. Pat. Nos. 3,615,858, and 3,655,450, entails providing a continuous layer of monovalent silver oxide in physical and electrical contact with a divalent silver oxide pellet. During assembly of the cell the cathode pellet is disposed against the inner surface of a cathode cup or collector whereupon the layer of monovalent silver oxide physically isolates the divalent silver oxide from contact with the cathode cup so that the sole electronic path for discharge of the divalent silver oxide is through the monovalent silver oxide layer.
United States patent applications Ser. No. 482,996 now U.S. Pat. No. 3,920,478 and Ser. No. 483,014 now U.S. Pat. No. 3,925,102 both of which were filed on June 25, 1974 in the name of A. Kozawa, disclose another approach to producing divalent silver oxide-zinc cells having a substantially unipotential discharge level even on low drain conditions. The cells use a positive electrode comprising divalent silver oxide housed in a positive electrode container having an upstanding wall and a closed end. Interposed between the positive electrode and the upstanding wall or bottom end of the cathode container is an oxidizable metal screen or ring, such as zinc, which functions to discharge the portion of the positive electrode contacting the container so as to produce a unipotential discharge on low drain conditions.
Although, it is thereby possible to produce a unipotential discharge using divalent silver oxide as an active material of a cell, a problem generally encountered is that during the initial discharge period an undesirably high internal resistance of the cell is manifested by a subnormal cell voltage which gradually increases with time to the expected unipotential voltage level. These voltage variations during the initial discharge of silver oxide-zinc cells, and other metal oxide cells whose cathodic discharge product is electronically conductive, are undesirable for many battery powered devices. A specific example would be battery powered watch applications where the accuracy of the watch movements can be impaired or the performance adversely affected by such voltage variations during intial discharge of the cell. Some proposals for overcoming this drawback in metal oxide miniature type cells would be to apply a conductive coating of a metal on the entire outer surface of the active cathode material of the cell as disclosed in U.S. Pat. No. 2,654,795. This reference also discloses that the conductive material used can vary from about 5% by weight of the active material and in the case where gold is employed as the conductive material, then an amount as little as fractions of 1% can be used.
U.S. Pat. No. 3,853,623 discloses divalent silver oxide cells wherein gold ions are added to the alkaline electrolyte of the cell in an amount of about 0.1 to about 10% by weight of the silver forming the active cathode material. The gold irons are added only on the cathode side of the separator and are employed to provide greater stability of the divalent silver oxide in the alkaline electrolyte of the cell and thus reduce gassing (O.sub.2). This patent also discloses that a gold compound can be added directly in the divalent silver oxide cathode, in combination with its addition to the electrolyte with the overall amount of gold in both locations falling within the range specified above.
Accordingly, it is an object of this invention to provide a metal oxide cell which has a thin layer of an electronically conductive material disposed between the separator and the positive electrode of the cell which is extended to contact the positive terminal of the cell so as to eliminate voltage variation during initial discharge of the cell.
Another object of this invention is to provide a miniature button silver oxide-zinc cell employing a thin layer of electronically conductive material at the interface of the separator and the positive electrode of the cell and wherein said conductive layer extends to contact the positive terminal of the cell.
Another object of this invention is to provide a thin electronically conductive layer on at least a portion of the surface of the active cathode material of a metal oxide cell such that said layer is in contact with the separator of the cell and is extended to contact the positive terminal of the cell.
Another object of this invention is to provide a thin electronically conductive layer on at least a portion of the surface of a separator of a metal oxide cell such that said conductive layer contacts the active material of the cell and is extended to contact the positive terminal of the cell.